1. Field of the Invention
The present invention is concerned with voltage-protected semiconductor bridge igniter elements, such elements having integral high voltage protection and, optionally, integral continuity testing capability.
2. Related Art
Semiconductor bridge ("SCB") elements, means to electrically connect them for the purpose of electrical activation, and the use of such devices as igniters to initiate explosives, are well-known in the art. Presently, both the SCB of U.S. Pat. No. 4,708,060, to Bickes, Jr. et al, issued Nov. 24, 1987, and the tungsten bridge SCB of U.S. Pat. No. 4,976,200, to Benson et al, issued Dec. 11, 1990, are manufactured with large metallized pads for electrical contact to the active area of the bridge. The disclosure of U.S. Pat. Nos. 4,708,060 and 4,976,200 is incorporated herein. The SCB chip generally is mechanically bonded to an attachment surface of a header or other element of an electro-explosive device ("EED"). Proper functioning of the SCB in a detonator requires intimate contact with an energetic material such as an explosive or pyrotechnic material, and thus demands an upright position for the chip; that is, the chip cannot be assembled with its active area positioned against the attachment surface, but its active area must face towards and contact the energetic material so that the active area is free to interact with the energetic material, i.e., to impart energy thereto to initiate the energetic material.
Voltage protection for SCB elements is a highly desirable safety attribute used to prevent accidental functioning of explosive devices in the presence of stray voltage. In general, high voltage protection prevents voltages below a threshold voltage ("V.sub.th ") from inducing current flow through the SCB. However, for voltages above V.sub.th, a current will flow through the SCB with sufficient amplitude to function the SCB and thereby generate a plasma that will initiate an explosive load placed in intimate contact with the SCB or serve some other desired function. Therefore, V.sub.th, is defined as the voltage that has to be exceeded before the SCB can be functioned. Such threshold voltages are generally in the range of from about 10 V to about 1000 V. It is known to provide high voltage protection for SCBs by various means; for example, spark gaps, near-intrinsic semiconductor films or substrates, and semiconductor diodes.
Spark gaps consist of a pair of encapsulated electrodes packaged in a gas or vacuum environment that are separated by a specific distance or "gap". The gap, in general, determines the breakdown or threshold voltage of the device. The "gap" must be accurately and consistently controlled during the assembly process to reduce the variability range of the threshold voltage. Such a highly controlled encapsulation and electrode spacing process is quite expensive. Another drawback of this spark gap approach is that the continuity of the SCB is not easy to monitor unless a voltage greater than the spark gap breakdown voltage is applied for a very short period of time. This situation of course causes an unsafe condition of flowing high current through the SCB.
Near-intrinsic semiconductor films or substrates may also be used for voltage protection. A near-intrinsic semiconductor can be designed to have a particular volume and a particular resistance value selected so that, upon the application of voltages in excess of V.sub.th, enough heat will be generated to create additional carriers that will lower the resistance of the device and eventually cause current flow. Such current flow is a consequence of the negative differential resistance that intrinsic semiconductors typically exhibit. Near-intrinsic semiconductor films require very low doping levels which are difficult to control because they depend mainly on two processes: i) thermal effects such as thermal diffusion and/or thermal annealing after, for example, ion implantation and, ii) high controllability in the impurity level during the in situ growth of the semiconductor film. In addition to the difficulty of controlling a low doping level, both the impedance and the size of the near-intrinsic element must be properly designed to permit the available energy to be rapidly delivered to heat and vaporize the film to create the plasma that will set off the explosive load.
Semiconductor diodes have been used to prevent current flow caused by applied voltages below the characteristic breakdown or threshold voltage that occur at the diode's junction when biased in the reverse mode. However, this protection is lost when the diode is biased in the forward mode, therefore making the diode-protected SCB a polarized device. To alleviate this polarization problem, back-to-back diodes may be used in series with the SCB to provide protection for the SCB in both polarities. However, a major drawback of this approach is the low doping level required for high breakdown voltages for a single diode and the need for different wafers (substrates) for different breakdown voltages. For example, a diode with 500 V breakdown voltage requires a substrate doping concentration of less than 10.sup.15 per cm.sup.3, which is impractical because of the difficulty of controlling such low concentrations of dopants. A solution which avoids the necessity for low doping levels is to use multiple low-voltage diodes interconnected in series with the SCB and in a back-to-back configuration. This, of course, results in a more elaborate design and use of a larger chip area. Another drawback of this back-to-back diode approach is that the continuity of the SCB is not easy to monitor unless a voltage greater than the diode breakdown voltage is applied for a very short period of time. This situation, of course, causes an unsafe condition of flowing high current through the SCB. There is, therefore, in addition to a need for an improved structure to provide high voltage protection for SCBs and the like, a need for an improved structure to enable continuity monitoring of the SCB device at various points during its manufacturing process and just prior to its use.